The present invention relates to the extraction of uranium values from uranium-containing materials. In a more specific aspect, the present invention relates to the extraction of uranium values from uranium-containing materials by the use of a leaching solution. Still more specifically, the present invention relates to the extraction of uranium values from mined ores or in situ from subsurface formations by the use of an aqueous alkaline leach solution containing an oxidant and, optionally, a catalytic material.
The importance of uranium as a source of energy is well established. Uranium occurs in a wide variety of subterranean strata such as granites and granitic deposits, pegmatites and pegmatite dikes and veins, and sedimentary strata such as sandstones, unconsolidated sands, limestones, etc. However, very few subterranean deposits have a high concentration of uranium. For example, most uranium-containing deposits contain from about 0.01 to 1 weight percent uranium, expressed as U.sub.3 O.sub.8 as is conventional practice in the art. Few ores contain more than about 1 percent uranium and deposits containing below about 0.1 percent uranium are considered so poor as to be currently uneconomical to recover unless other mineral values, such as vandium, gold and the like, can be simultaneously recovered. However, in most cases, concentrations of the latter materials are too low to improve the economics to any great extent and techniques for recovering the uranium often are not well adapted to the recovery of other valuable minerals.
There are several known techniques for extracting uranium values from uranium-containing materials. One common technique is roasting of the ore, usually in the presence of a combustion supporting gas, such as air or oxygen, and recovering the uranium from the resultant ash. However, the present invention is directed to the extraction of uranium values by the utilization of aqueous leaching solutions. There are two common leaching techniques for recovering uranium values, which depend primarily upon the accessibility and size of the subterranean deposit. To the extent that the deposit containing the uranium is accessible by conventional mining means and is of sufficient size to economically justify conventional mining, the ore is mined, ground to increase the contact area between the uranium values in the ore and the leach solution, usually less than about 14 mesh but in some cases, such as, limestones, to nominally less than 325 mesh, and contacted with an aqueous leach solution for a time sufficient to obtain maximum extraction of the uranium values. On the other hand, where the uranium-containing deposit is inaccessible or is too small to justify conventional mining, the aqueous leach solution is injected into the subsurface formation through at least one injection well penetrating the deposit, maintained in contact with the uranium-containing deposit for a time sufficient to extract the uranium values and the leach solution containing the uranium, usually referred to as a pregnant solution, is produced through at least one production well penetrating the deposit.
The most common aqueous leach solutions are either aqueous acidic solutions, such as sulfuric acid solutions, or aqueous alkaline solutions, such as sodium carbonate and/or bicarbonate.
While aqueous acidic solutions are normally quite effective in the extraction of uranium values and act quite rapidly in the extraction of the uranium values, the volumes of acid consumed are usually quite high, thus making the use of aqueous acidic solutions relatively expensive. In addition, aqueous acidic solutions generally cannot be utilized to extract uranium values from ores or in situ from deposits containing high concentrations of acid-consuming gangue, such as limestone. On the other hand, aqueous alkaline leach solutions are either not as effective in the extraction of uranium values and/or extract the uranium values at a rate which is too slow to be economically justified.
The uranium values are conventionally recovered from acidic leach solutions by techniques well known in the mining art, such as direct precipitation, selective ion exchange, liquid extraction, etc. Similarly, pregnant alkaline leach solutions may be treated to recover the uranium values by contact with ion exchange resins, precipitation, as by adding sodium hydroxide to increase the pH of the solution to about 12, etc.
As described to this point the extraction of uranium values is dependent strictly upon the economics of mining versus in situ extraction and the relative costs of acidic leach solutions versus alkaline leach solutions. However, this is an oversimplification, to the extent that only uranium in its hexavalent state can be extracted in either acidic or alkaline leach solutions. While some uranium in its hexavalent state is present in mined ores and subterranean deposits, the vast majority of the uranium is present in its valence states lower than the hexavalent state. For example, uranium minerals are generally present in the form of uraninite, a natural oxide of uranium in a variety of forms such UO.sub.2, UO.sub.3, UO.U.sub.2 O.sub.3 and mixed U.sub.3 O.sub.8 (UO.sub.2.2UO.sub.3), the most prevalent variety of which is pitchblende containing about 55 to 75 percent of uranium as UO.sub.2 and up to about 30 percent uranium as UO.sub.3. Other forms in which uranium minerals are found include coffinite, carnotite, a hydrated vanadate of uranium and potassium having the formula K.sub.2 (UO.sub. 2).sub.2 (VO.sub.4).sub.2.3H.sub.2 O, and uranites which are mineral phosphates of uranium with copper or calcium, for example, uranite lime having the general formula CaO.2UO.sub.3.P.sub.2 O.sub.5.8H.sub.2 O. Consequently, in order to extract uranium values from mined ores and subsurface deposits with aqueous acidic or aqueous alkaline leach solutions, it is necessary to oxidize the lower valence states of uranium to the soluble, hexavalent state. It has heretofore been suggested that air, oxygen and other known oxidants be added to the leach solution in order to accomplish the oxidation of the uranium to its hexavalent state. Obviously, a major factor in the utilization of oxidants in leach solutions is the cost of the oxidant itself. While air would appear to be the least expensive oxidant to utilize, certain difficulties are encountered, to the extent that insufficient air can be dissolved in the leach solution at atmospheric pressure thereby rendering the extraction process rather inefficient. While adding air to the leach solution under pressure will obviously incease the volume of air available for oxidation and improve the ultimate recovery of uranium values and the rate of recovery, the compression equipment necessary, for example, to add air under pressures of about 1000 to 2000 psi or higher for ore leaching or in situ extraction, necessarily adds to the cost of the operation. Of the other known oxidants which have been suggested in the prior art, the oxidant itself becomes a major cost factor. For example, stoichiometric quantities of most of the prior art oxidants range anywhere from about 10 to 80 pounds or more of oxidant per ton of ore treated. However, even aside from cost, the utilization of oxidants in leach solutions has a number of other drawbacks. For example, the relative effectiveness of various known oxidants varies widely. Further, a number of known oxidants are unstable, decompose, are otherwise lost during use, or lose their effectiveness for one reason or another. Finally, there appears to be no certain way of predicting what materials will act as oxidants in combination with which leach solution. For example, certain oxidants useful in aqueous acidic solutions are not useful in aqueous alkaline solutions, certain oxidants which are effective with certain acids, forming an aqueous acid solution, are not effective with other acids and certain oxidants effective with certain alkaline materials, making up an alkaline leach solution, are not effective with other alkaline materials.
In order to reduce the quantity of oxidant necessary, increase the ultimate effectiveness of the oxidants and/or increase the rate of extraction of the uranium values, it has been suggested that catalytic amounts of certain materials be added to alkaline leach solutions containing oxidants. Some of these catalytic materials are themselves oxidants when utilized in stoichiometric quantities but also act as catalysts when utilized in catalytic quantities well below stoichiometric amounts. In most cases the catalytic materials are materials adapted to supply ions of metals capable of existing in high and low valence states. The latter has led to the theory that the catalyst enters into a redox reaction in which the oxidant of the leach solution oxidizes the metal ion to its higher valence state, the metal ion in its higher valence state oxidizes the uranium and is thereby itself reduced in valence and the cycle continues with the oxidant oxidizing the catalytic ion to its higher valence state, etc. Several other theories have also been advanced to explain why a particular catalytic material or group of catalytic materials functions as a catalyst. However, none of the theories concerning the role of the catalytic materials appears to be applicable to all catalytic materials which have been found effective. Consequently, there appears to be no basis for predicting a particular material will be effective as a catalyst in an alkaline leach solution containing an oxidant. In addition, the utilization of catalytic materials is fraught with the same uncertainties as the utilization of oxidants. Specifically, materials which should be effective as catalysts in accordance with a particular theory are often ineffective, unstable in the leach solution or ineffective in combination with a particular alkaline material, a particular oxidant or a combination of a particular alkaline material and a particular oxidant.